Change __builtin_unreachable to __builtin_trap (or infinite loop) if only thing in...

Change __builtin_unreachable to __builtin_trap (or infinite loop) if only thing in function [PR109267]

When we have an empty function, things can go wrong with
cfi_startproc/cfi_endproc and a few other things like exceptions. So if
the only thing the function does is a call to __builtin_unreachable,
let's replace that with a __builtin_trap instead if the target has a trap
instruction.  For targets without a trap instruction defined, replace it
with an infinite loop; this allows not to need for the abort call to happen
but still get the correct behavior of not having two functions at the same
location.

The QOI idea for basic block reorder is recorded as PR 120004.

Changes since v1:
* v2: Move to final gimple cfg cleanup instead of expand and use
      BUILT_IN_UNREACHABLE_TRAP.
* v3: For targets without a trap defined, create an infinite loop.

Bootstrapped and tested on x86_64-linux-gnu.

	PR middle-end/109267

gcc/ChangeLog:

	* tree-cfgcleanup.cc (execute_cleanup_cfg_post_optimizing): If the first
	non debug statement in the first (and only) basic block is a call
	to __builtin_unreachable change it to a call to __builtin_trap or an
	infinite loop.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_trap): New proc.
	* g++.dg/missing-return.C: Update testcase for the !trap case.
	* gcc.dg/pr109267-1.c: New test.
	* gcc.dg/pr109267-2.c: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>